The present invention relates to a method of making material for shadow masks to be incorporated in cathod ray tubes for colour TV sets, which is to provide a production of ultra low carbon Al-killed steel sheet having excellent photo-etching and press forming properties.
For making shadow masks, rimmed steel is in general used (including capped steel), and passed through a series of processes of coiled cold rolled rimmed steel-ordinary annealing or decarburization annealing-temper rolling-re-cold rolling-photo-etching-cutting-annealing (final annealing)-levelling-pressing-surface treatment-setting up.
In such a process, when conventional rimmed steel is used undesirable defects become apparent during photo-etching due to non-metallic inclusions which are unavoidable in the rimmed steel. With respect to the ordinary annealed material, bad etching is caused by coarse carbide existing in the material, or difficulties arise adversely affecting the precision of holes, at pressing because of the hardness of the material.
The inventors have already proposed, in Japanese Patent Application No. 53-133,245, a method for making shadow masks using low carbon Al-killed steel.
The present invention is to provide a further improvement of the photo-etching property and the press-formability than those of the steel of said patent application. That is, in the present invention the coil of an ordinary cold rolled Al-killed steel is forcibly decarburized in open coil annealing (referred to as "OCA" hereinafter) until solute carbon is decarburized up to an amount where the amount cannot be quantitatively confirmed by means of usual methods, that is, until the quench aging index (referred to as "QAI" hereinafter) becomes less than 3.0 Kg/mm.sup.2.
Herein "QAI" is specified as follows: ##EQU2## wherein, W1: load (Kg) giving 10% tensile strain to the said decarburized material having been soaked at temperature of 500.degree. C. for 10 minutes and subjected to a water cooling
S: cross sectional area (mm.sup.2) of a test piece when giving said 10% tensile strain PA1 W2: yield point load (Kg) provided by the said strain effected material aged at temperature of 100.degree. C. for 4 hours.